The present invention relates to a creep torque and lock-up control for an automatic transmission.
Laying-open Japanese patent application No. 59-75840 (U.S. counterpart: U.S. Ser. No. 543,838 (now U.S. Pat. No. 4,542,665), filed Oct. 20, 1983 (EP counterpart: European patent application EP No. 83110546.5, filed Oct. 21, 1983) discloses a control device for a hydraulic automatic clutch used as a starting clutch in an automatic transmission in the form of a continuously variable transmission. According to this control, hydraulic fluid pressure supplied to a starting clutch is controlled by a start adjustment valve such that engagement state of the starting clutch taking place at engine idling is kept at a predetermined state. With this control, the starting clutch is controlled such that it produces a predetermined creep torque at engine idling.
However, in the case of the use of an automatic transmission employing a hydrokinetic torque transmitting unit provided with a lock-up mechanism, it is also necessary to control the lock-up mechanism in such a manner that it is engaged or released upon satisfying a predetermined condition in addition to controlling the forward/reverse clutch in the above mentioned manner so as to cause it to produce a predetermined creep torque. To accomplish this mission, another solenoid is required which shifts the lock-up control valve based on, for example, vehicle speed signal and throttle opening degree signal. This causes an increase in the installation space of the control device and an increase in manufacturing cost.
An object of the present invention is to solve the above mentioned problem and to provide a control device for an automatic transmission which can execute two different controls without any substantial increase in the number of hardware components, thus avoiding any increase in the installation space of the control device and in manufacturing cost.